In the semiconductor fabrication process, a square cross-sectional or rectangular cross-sectional container made of a plastic material is frequently used to transport articles. These articles may include silicon wafers, reticles or other substrates used for electronic structures. A reticle is a transparent ceramic substrate that is coated with a metallic layer forming a pattern for an electronic circuit. It is generally used in an imaging step during a photolithographic process wherein a pattern of a circuit is reproduced on the surface of an electronic substrate, i.e., such as a wafer surface.
A reticle can be constructed of any suitable transparent ceramic materials. However, one of the most commonly used material is quartz or silicon dioxide. A quartz reticle can be readily coated with a chrome layer at selective areas to reproduce an electrical circuit. The chrome metal layer may be formed by either a pure chromium or a chromium alloy. During a photolithographic imaging process, a light source is projected from one side of the reticle that is coated with the pattern such that the pattern can be reproduced on the surface of a wafer which is positioned on the opposite side of the reticle. The pattern for the electronic circuit coated on the reticle is frequently laid out in a 5.times. magnification. The true dimensions of the electronic circuit reproduced on the wafer surface can be obtained by suitably adjusting the optical lenses situated between the reticle and the wafer. Metallic coatings other than chrome may also be coated on the surface of the reticle for the circuit lay-out. However, chrome has been found to be an ideal material for its appearance of a brownish tone and its ease of identification by human eyes.
In a semiconductor fabrication facility, static electricity or electrostatic discharge frequently develops on surfaces of articles made of insulating materials when they are touched or rubbed by other insulating materials such as insulating gloves. The electricity is produced based on a triboelectricity theory. The discharge of the static electricity to machines and to human operators can cause damages to semiconductor wafers and process tools. Sometimes, it may even cause injury to a machine operator. In a semiconductor fabrication facility, it is therefore necessary to control ESD by grounding the machines, by controlling the relative humidity, or by building walls and floor coverings with slightly conductive materials such that electrical charges can be routed to ground. When the triboelectricity is suitably controlled, the control of dust and particulate contamination is also enhanced. For instance, the metal racks, pipe lines, cabinets, cables and rails are normally grounded in a facility to an equal potential bar or to a planar ground. The metal pedestals of the raised floor are then connected to the planar ground under the raised floor. To further enhance ESD protection, the metal framework of the clean room wall systems are also connected to the planar ground. Air ionization systems are frequently installed at selected locations in a fabrication facility, to provide additional ESD control.
Despite the elaborate efforts spent in grounding process machines and various facilities, ESD damages still occur in a fabrication facility. A typical example is the occurrence of ESD when an insulating material is shipped or transported in a container made of another insulating material. For instance, when a reticle is transported from a storage facility to a photolithography machine in a container, i.e., a pod, that is normally constructed of a thermoplastic material. Since the reticle itself is an insulating material, i.e., a quartz or other silicon dioxide materials coated with a chrome coating, when the pod is handled by machine operators wearing insulating gloves, the static charge on the pod drastically increases due to friction generated between two insulating articles. Since the pod is not equipped with an anti- electrostatic device, high static electricity is generated on the surface of the pod. For instance, it has been confirmed that the static electricity field generated on a pod surface increases from 0.1 KV/inch to nearly 15 KV/inch when a polycarbonate pod is rubbed with PVC gloves. Such a high static electricity build-up on the surface of the pod immediately causes a electrostatic discharge between the pod and the reticle contained therein. When ESD occurs between the pod and the reticle, the pattern on the reticle surface is usually damaged to such an extent that it can no longer be used for imaging. Conventional air ionization devices installed at a fabrication facility are not useful for preventing such ESD damages.
Others have proposed techniques for controlling or minimizing ESD damages to reticles carried in plastic containers. For instance, anti-electrostatic-type plastic materials, such as Bayon.RTM. has been used for the construction of the pod. However, due to its high cost, this type of anti-electrostatic plastic material cannot be widely utilized in a fabrication facility. Still others have proposed the use of gloves that are made of a conductive material such as Propex.RTM. such that the generation of electrostatic discharge can be avoided. The high cost of the Propex.RTM. gloves prohibit its broad usage in the processing industry.
It is therefore an object of the present invention to provide a method for preventing ESD damages to an insulating article stored in a container made of another insulating material that does not have the drawbacks or shortcomings of the conventional storage methods.
It is another object of the present invention to provide a method for preventing ESD damages to an insulating article stored in a container made of another insulating material that does not require the use of high cost conductive plastic materials or gloves.
It is a further object of the present invention to provide a method for preventing ESD damages to an insulating article stored in a container made of another insulating material which only requires a re-design of the configuration of the container.
It is another further object of the present invention to provide a method for preventing ESD damages to a reticle coated with a chrome pattern and stored in a thermoplastic container such that the chrome pattern on the quartz reticle is not damaged by the occurrence of electrostatic discharge.
It is still another object of the present invention to provide a method for preventing ESD damages to a reticle coated with a chrome pattern and stored in a polycarbonate container by re-designing the height of the container.
It is yet another object of the present invention to provide a method for preventing ESD damage to a reticle coated with a metallic pattern and stored in a thermoplastic container by increasing the distance between the top surface of the reticle and the top lid of the container to larger than a threshold value such that ESD does not occur.
It is still another further object of the present invention to provide a method for calculating a minimum safe distance between the top surface of a reticle stored in a pod and the top lid of the pod such that electrostatic discharge is avoided and the chrome pattern coated on the reticle is not damaged.